Volume compression circuit



y 3, 1952 DE. ROBERTS 2,596,510

VOLUME COMPRESSION CIRCUIT Filed Aug. 25, 1947 \2 u I T T n 3L :1 I5

8 I 20 I6 INPUT o- 1 l4 l9 17 OUTPUT- Y n l v LI Invenror David E. Roberts By Patented May 13, 1952 UNITED STATES David E. Roberts, Chicago, 111., Sign; a; Motor. '"ola, Inc., Chicago, Ill., a. corporation of Illinois;

east? 23, e? na e-awe- 'Acqordinglm'the. circuit is so designed that the bias" Voltage supplied to the control grid of the controlled tube will rise very rapidly when there isfa. sudden rise inthe signal level. In conventionalvolume compressors the sudden rise in bias Voltage produces a steep waveiront transient which isjiob'jectionable to the listener, resulting in a momentary cur-err of the signal following each str ng signal peak. It is not feasible to filterioutthifitransient by placing a resistance-ea paeitaiice network betvvee'n the rectifier and the control grid, sincethe .tirne'delay thus introduced would slowiup the automaue volume control jac- Another problem which arises in the design of volumecjornpression circuits is the tendency of the screen grid potential of the controlled tube to'vary with changes in the control bias, Such aotionjcausesftheoperating point of the screen grid potential to be shifte'd outof theregion of maximunifcurvature 'ofthe transfer character? jstic therebvldimihishingthe automatic volume V c t actio t as been proposed o e d thisfldiificulty bythe use of a bleeder resistor betvv'ee 11 grid and ground, which minimizes the screen voltage fluctuations resulting from changes in screen current. This expedient, however; is ohjeetioneghle because the bleeder current powerl and the circuit does not accomif "ordinary'variations in the oper'a't esar'nongiridividual'tubes of the propose to accomplsh this bylimiting the'amplitilde O f the transient through the medium of a clipping circuit. I'r ave'observed that theemphtude of the transient is many times as'greatas e or i s, (o1. m w

eitree t sne amplifier tube in the eutofnatie voliiniiiofittdl A further object of the present invention is to overcome thediffictiltyfusual1y"ei 1erif1ceii win-i unstable screen gnu Voltages volu'xnecon1pres= s'ors by means designedTto "introduce 'a7buck1fig voltage in'series Witnthe' seree'n' dropping resistor voltage, thereby to stabilize the 'sereen-grid volt age. Mt, H s U Lt -x. a "r.

Ejurther objects, features and advantages of this invntihtiWillhecom ap are t fr'oifith'e for: rowing description when tal'rieriinconnectiofivvith the accompanying d'raii in'g' "when illustrates uz n the p inciples the'inveritioh'fi' h In ca iflryin iittheabovstatdobjeots 0f the inventionflf arranglefa c'li tube'so tlliat'it'is responsive, to" the plate voltage jof the controlled circuit. This clipper tube draws its plate cu voltage or bias' is applied through a grid dropp'ing resistord .to the control 'gridiiofithe tube Thec'apaoitor t and resistor 1 togetherlhavea time constant sufficiently great Ito hold the bias btweenlsyllablestor signalcr'e's ts. input Sigrial, which aopneathroogn academia-gaspedtrol'grid5","issup p' the c ntra b1 .""Th"plalt Qdf "the tu'b'e't draws its current rom a uitafile plat voltag seorce throughan isolating resistor "Ill and the plate or load resistor II. "The carpen er l2 is'fo'r isolation iir ese l' nl Th lte' e n ov i l fi if th plate voltage of tube I is fed through a coupling capacitor 53 to the input terminal of the amplifier 2, which may comprise one or more stages of amplification as desired.

The clipper stage includes a high-mu triode I4 which is connected into the circuit in controlling relation to the pentode I, as shown in the drawing. A voltage divider, comprising the series-connected resistors I5, I6, I1 and I8, is connected between the plate and cathode terminals of the tube I. The plate I9 ofthe tube I4 is connected to the junction of the resistors I5 and I5. The screen grid 29 of the tube I is connected to the junction of the resistors I6 and I! and is bypassed to ground by a capacitor 2|. The con trol grid 22 of the tube I4 is connectedto the junction of resistors I! and I8 and is by-passed to ground by a capacitor 21. The cathode 23 of the tube I4 is connected to the junction of resistors 24 and 25, which are connected in series between the plate voltage source and ground to afiord a voltage divider for the purpose of supplying the proper cathode bias for the tube I4.

The by-pass capacitor 26 is connected in parallel with the resistor 25.

The triode I4 is biased to operate in the low plate-current, high plate-resistance region, with the result that normally it does not greatly load the plate circuit of the pentode I. The presence of the by-pass capacitors 2| and 2'! prevents any degeneration of signal voltages in consequence of the presence of the triode I4. It should be noted that the screen grid of the tube I draws its current through the plate resistor I5 oi the tube I4 and also through the resistor I6 in series therewith. Likewise, the tube I4 draws its plate current through the plate resistor II of thetube I.

The characteristics of the triode [4 are such that if the plate voltage of the pentode I should rise above its normal value in an amount greater than, say, 20 volts, the tube I4 operates on the V steep portion of its plate characteristic, with the nal voltages combined with the D. C. plate voltage shifts of the tube I resulting from automatic volume control action remain well below 20 volts and are not afiected by the action of the tube It. Negative plate voltage swings of the tube I tend to be slower and do not cause audible transients; hence, no control thereof is required.

It has been mentioned hereinabove that the triode or clipper tube I 4 stabilizes the screen grid voltage of the tube I. When the grid of the pentode I receives a negative AVC voltage, the screen grid voltage tends to rise, due mainly to the reduction in screen current through the screen dropping resistors I5 and I6. Such a rise,

however, raises the control grid voltage of the triode I4, causing an increase in the triode plate current through resistor I5. This produces a bucking voltage across resistor I5 which tends to restore the voltage of the screen grid 20 to its.

shifted out of the critical region of maximum toring system, although obviously it is capable of other uses as well.

While the invention has been specifically described with respect to a preferred embodiment thereof, it is to be understood that modifications and alterations can be made therein which are within the full intended scope of this invention as defined in the appended claims.

I claim: 7

1. In a volume compression circuit for an amplifier having a controlled stage and automatic biasing means for said stage, a variable-mu pentode in said controlled stage, a plate resistor through which said pentode draws its plate current, a plurality of resistors connected in series across the plate and cathode terminals of said pentode, said pentode having a screen grid arranged to draw current through certain of said biasing means for said stage, a variable-mu pentode in said controlled stage, a plate resistor through. which said pentode draws its plate current, resistance means connected across the plate and cathode terminals of said pentode, said pentode having a screen grid connected to an intermediate point on said resistance means to draw current through a portion thereof, and a triode having plate and grid electrodes connected to other points on said resistance means, said plate of said triode being connected to said resistance means at a point between the connection, to said plate of said pentode, and said intermediate point and said grid oi. said triode being connected to said resistance means at a point between the connection to said cathode of said pentode and said intermediate point, whereby said triode plate current is drawn through said pentode plate resistor and through a portion of said resistance means which is in common with said screen grid, said triode normally operating with low plate current and high plate resistance but being responsive to pentode plate voltage exceeding a givenamount and to high pentode. screen grid voltage for drawing high plate current thereby to limit such pentode plate and screen grid voltages.

3. In an amplifier having a controlled amplification stage and means for automatically applying a volume control voltage to said controlled stage from a subsequent stage of theamplifier to hold the output of said controlled stage within a desired range, and in which the volume control Opera-V voltage may have steep wave fronts which tend to cut ofi said controlled amplification stage, a system for limiting the effect of said steep wave fronts including in combination, an amplifier tube in said controlled stage including a plate and a cathode, first resistance means connecting said plate to a source of potential and operating to drop the voltage of said plate below the voltage of said source when current is drawn therethrough and allowing the voltage of said plate to approach the voltage of said source when said amplifier tube is cut off, voltage divider means connected across said plate and said cathode of said amplifier tube, a triode tube having a plate and agrid, said plate of said triode tube being connected to a first intermediate point on said voltage divider means so that a portion of the voltage at said plate or said amplifier tube is applied to said plate of said triode tube, said grid of said triode tube being connected to a second intermediate point on said voltage divider means between said first intermediate point thereon and the end thereof connected to said cathode of said amplifier tube and providing a bias for said triode tube, said first resistance means and the portion of said voltage divider means between said plate 01' said amplifier tube and said first intermediate point thereon providing a path for the plate current of said triode tube, said first and second intermediate points on said voltage divider means applying potentials to said plate and said grid of said triode tube so that the plate current or said triode tube is low when said amplifier tube is operat nawith the output thereof within the desired range, with the voltage applied to said plate and grid of said triode from said voltage divider means increasing when the plate voltage of said amplifier? tube increases above the desired range to cause substantially increased plate current through said triode and through said resistance means, and thereby increasing the voltage drop across said resistance means to reduce the plate voltage of said amplifier tube.

DAVID E. ROBERTS.

REFERENCES CITED The following references are of record in the file of this patent: 4

UNITED STATES PATENTS Number Name Date 2,092,496 Branson Sept. 7, 1937 2,179,263 Koch Nov. 7, 1939 2,214,608 Bull Sept. 10,1940 2,315,042 Boucke Mar. :30, 1943 2,360,466 Bedford Oct. 17, 1944 2,363,985 Moser "Nov. :28, 1944 2,383,309 Hathaway Aug. 21, 1945 2,397,830 Bailey Apr. 2, 1946 2,420,248 Koch May 6, 1947 

